The invention relates to a disconnecting-gear device for a timepiece gear train, this gear train actuating a minute wheel from a driving member and including a plurality of wheels and pinions arranged in a chain and meshing with each other, this chain including a wheel and pinion able to be disconnected from one of the other wheel and pinions by the action of a lever controlled by a time-setting stem to interrupt the chain and stop the minute wheel.
A disconnecting-gear device answering the generic definition of the above paragraph is known. A description thereof is given, for example, in document EP-B-O 261 243 (=U.S. Pat. No. 4 862 434).
In this document, the disconnecting-gear device is also disposed between a driving element and a minute wheel. It is a wheel and pinion able to be disconnected from the minute wheel, this wheel and pinion being the wheel following the centre-wheel, known in French by the name of &lt;&lt;roue moyenne&gt;&gt;, hereinafter the &lt;&lt;third wheel&gt;&gt;, but in reality including a wheel and a pinion with a common shaft. The wheel is meshed with a second centre-pinion, whereas the pinion is meshed with the minute wheel. One of the pivots of this wheel and pinion, which will be called the third pivot, rotates in a drill hole made in a bridge, whereas the other pivot rotates in a drill hole made in a lever controlled by a time-setting stem. When the time-setting stem is pulled out, the shaft of the third wheel and pinion inclines in such a way that its pinion leaves the toothing of the minute wheel. The hour and minute display is thus stopped, the hands being then able to be set by manipulating the stem via the motion-work.
This disconnecting-gear device has the drawback of increasing the thickness of the timepiece, since it is clear that space must be provided for the third wheel in the inclined state. On the other hand, since the pinion of the third wheel meshes directly with the minute wheel, it is also clear that during a gear reconnecting the minute hand can lead to a display error, the significance of this error depending upon the position of the pinion toothing with respect to the wheel toothing at the moment of gear reconnection. If the wheel has 60 teeth, the error may be as much as plus or minus a half minute. This error may be calculated by the following formula where E is the error and n is the number of teeth of the minute wheel: ##EQU1##
In the case wherein n=60, E=.+-.3.degree.=approximately.+-.1/2 minute.